ffclock • man page

ffclock • man page

ffclock (2)

Leading comments

Copyright (c) 2011 The University of Melbourne
All rights reserved.
This documentation was written by Julien Ridoux at the University of
Melbourne under sponsorship from the FreeBSD Foundation.
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions
are met:
1. Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimer.
2. Redistributions in binar...

(The comments found at the beginning of the groff file "man2/ffclock.2freebsd".)

LIBRARY

SYNOPSIS

DESCRIPTION

The ffclock is an alternative method to synchronise the system clock.
The ffclock implements a feed-forward paradigm and decouples the timestamping
and timekeeping kernel functions.
This ensures that past clock errors do not affect current timekeeping, an
approach radically different from the feedback alternative implemented by the
ntpd daemon when adjusting the system clock.
The feed-forward approach has demonstrated better performance and higher
robustness than a feedback approach when synchronising over the network.

In the feed-forward context, a
timestamp
is a cumulative value of the ticks of the timecounter, which can be converted
into seconds by using the feed-forward
clock estimates

The
Fn ffclock_getcounter
system call allows the calling process to retrieve the current value of the
feed-forward counter maintained by the kernel.

The
Fn ffclock_getestimate
and
Fn ffclock_setestimate
system calls allow the caller to get and set the kernel's feed-forward clock
parameter estimates respectively.
The
Fn ffclock_setestimate
system call should be invoked by a single instance of a feed-forward
synchronisation daemon.
The
Fn ffclock_getestimate
system call can be called by any process to retrieve the feed-forward clock
estimates.

The feed-forward approach does not require that the clock estimates be retrieved
every time a timestamp is to be converted into seconds.
The number of system calls can therefore be greatly reduced if the calling
process retrieves the clock estimates from the clock synchronisation daemon
instead.
The
Fn ffclock_getestimate
must be used when the feed-forward synchronisation daemon is not running
Po see
Sx USAGE
below
Pc .

The clock parameter estimates structure pointed to by
Fa cest
is defined in
In sys/timeffc.h
as:

RETURN VALUES

ERRORS

A user other than the super-user attempted to set the feed-forward clock
parameter estimates.

USAGE

The feed-forward paradigm enables the definition of specialised clock functions.

In its simplest form,
Fn ffclock_getcounter
can be used to establish strict order between events or to measure small time
intervals very accurately with a minimum performance cost.

Different methods exist to access absolute time
Po or
Qq wall-clock time
Pc tracked by the ffclock.
The simplest method uses the ffclock sysctl interface
kern.ffclock
to make the system clock return the ffclock time.
The
clock_gettime2
system call can then be used to retrieve the current time seen by the
feed-forward clock.
Note that this setting affects the entire system and that a feed-forward
synchronisation daemon should be running.

A less automated method consists of retrieving the feed-forward counter
timestamp from the kernel and using the feed-forward clock parameter estimates
to convert the timestamp into seconds.
The feed-forward clock parameter estimates can be retrieved from the kernel or
from the synchronisation daemon directly (preferred).
This method allows converting timestamps using different clock models as needed
by the application, while collecting meaningful upper bounds on current clock
error.

SEE ALSO

HISTORY

Feed-forward clock support first appeared in
Fx 10.0 .

AUTHORS

An -nosplit
The feed-forward clock support was written by
An Julien Ridoux Aq Mt jridoux@unimelb.edu.au
in collaboration with
An Darryl Veitch Aq Mt dveitch@unimelb.edu.au
at the University of Melbourne under sponsorship from the FreeBSD Foundation.